Exhaust gas conversion method using thermally stable zeolites

ABSTRACT

A system and method are disclosed for converting exhaust gas which comprises NO x , CO, and hydrocarbons, to innocuous products. Exhaust gas is contacted in undiverted flow with a zeolite suitable for adsorption of hydrocarbons, and with a main catalyst suitable for conversion of NO x , CO, and hydrocarbons to innocuous products. Adsorption occurs when the temperature of the zeolite is suitable therefor. Conversion occurs when the temperature of the main catalyst is suitable therefor. The zeolite and main catalyst are connected directly to one another, that is, in-line, without by-pass valving. The zeolite can be Y zeolite, Beta, ZSM-5, and combinations thereof, and has a SiO 2  to Al 2  O 3 , mole ratio of no greater than about 200, and is ion exchanged with a metal which can be rare earth, chromium, and combinations thereof.

This application is a divisional of Ser. No. 08/183,472, filed on Jan.18, 1994, now U.S. Pat. No. 5,447,694.

This invention relates to an exhaust gas conversion method and apparatusin which a thermally stable zeolite placed in-line with the mainconverter, connected directly to the main converter without by-passvalving.

BACKGROUND OF THE INVENTION

Internal combustion engines emit a large amount of unburned hydrocarbonsduring cold engine start-up. In fact, a substantial fraction of thetotal emitted hydrocarbons have been found to occur during the firstminutes due to the uncombusted hydrocarbons in the rich fuel mixture.

Low molecular weight hydrocarbons are especially troublesome aspollutants because they form ozone. Emission standards for low molecularweight hydrocarbons are becoming more stringent. Zeolites withrelatively low SiO₂ to Al₂ O₃ mole ratios are very suitable foradsorbing low molecular weight hydrocarbons. However, their thermalstablity is relatively low.

Release of hydrocarbons after start-up of an engine poses a specialproblem because at this point the temperature of the exhaust gas and themain catalyst are not high enough for conversion to innocuous productsin the presence of conventional catalysts. The catalysts utilized incatalytic converter systems are generally ineffective at ambienttemperature and must reach high temperatures, often in the range of300°-400° C. before they are activated.

One approach to reducing cold start emissions is to temporarily adsorbhydrocarbons on zeolites. These methods involve using the zeoliteadsorber in the exhaust stream in a "by-pass" mode. That is, the zeoliteadsorber is exposed to the cold start exhaust, but once the mainconverter reaches its light-off temperature (about 300°-400° C.) asystem of valves sends the exhaust directly to the main converter,by-passing the adsorber.

U.S. Pat. No. 4,985,210 relates to an exhaust gas purifying apparatusemploying a 3-way catalyst which has either a Y-type zeolite or amordenite as an adsorbent at the upstream side of the catalyticconverter so that when the exhaust gas temperature is not higher than aspecific temperature, a harmful component is adsorbed by means of theadsorbent, whereas when the exhaust gas temperature exceeds the specifictemperature the harmful component is desorbed from the adsorbent and isintroduced into the catalytic converter. Further, an activated carbontrapper and a by-pass are provided in parallel at the upstream side ofthe adsorbent so that the flow paths of the exhaust gas are selectivelyswitched from one to the other in accordance with the level of theexhaust gas temperature.

U.S. Pat. No. 5,125,231 relates to an engine exhaust system designed toreduce hydrocarbon emissions having first and second catalyticconverters. Exhaust gas is conveyed to the first and second convertersdepending on the temperature. Hydrocarbons are adsorbed in the secondconverter at lower temperatures, and desorb at higher temperatures andare conveyed to the first converter. A system of thermostaticallycontrolled valves is used to convey the exhaust gas to the properconverter depending on the temperature.

In the above described by-pass modes of operation, the thermal demandson the zeolite in the adsorber are relatively simple. However, thedemands on the valve system are quite significant, in that it mustfunction reliably for at least 50,000 miles of driving, or even higher.In either situation, an expensive system of valves will be required,with safety and reliability always a concern.

Therefore, there is a need to develop an exhaust gas purification systemwhich will effectively remove pollutants both at cold start and afterwarm-up, without the need for extra valving.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided amethod for converting exhaust gas which comprises NO_(x), CO, andhydrocarbons, to innocuous products. The method comprises contacting theexhaust gas in undiverted flow with a zeolite suitable for adsorption ofhydrocarbons, and with a main catalyst suitable for conversion ofNO_(x), CO, and hydrocarbons to innocuous products, to cause adsorptionwhen the zeolite is at a temperature suitable therefor, and to causeconversion when the main catalyst is at a temperature suitable therefor.The zeolite can be Y zeolite, Beta, ZSM-5, and combinations thereof. Thezeolite has a SiO₂ to Al₂ O₃ mole ratio of no greater than about 200 andis ion exchanged with a rare earth, chromium, or combinations thereof.

In accordance with another aspect of the invention, there is provided anapparatus suitable for converting exhaust gases from an exhaust gasgenerating source, to innocuous products. The apparatus comprises azeolite unit as described above for adsorbing hydrocarbons, a maincatalyst unit for converting NO_(x), CO, and hydrocarbons to innocuousproducts positioned downstream from the zeolite unit, and conduit meansfor connecting the units with one another and with the exhaust gassource, and for providing undiverted passage of the exhaust gas from theexhaust gas source sequentially to and through the units.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plot showing the surface area of of Y zeolite, (H⁺), and ofCr-Y zeolite after exposure to 1000° C. as compared with the surfacearea of the Y zeolite which has not been exposed to 1000° C. (25° C.).

FIG. 2 is a schematic diagram showing the zeolite of the presentinvention connected in-line with a main catalyst in which they share asingle cannister.

FIG. 3 is a schematic diagram showing the zeolite of the presentinvention connected in-line with a main catalyst, with each being in aseparate cannister.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a method and apparatus for converting exhaustgas from an exhaust gas generating source, to innocuous products. Byinnocuous products is meant those that are generally considered to beharmless to health and the environment such as, CO₂, N₂, H₂, and water.The invention involves use of a thermally stable zeolite connected"in-line" with a main catalyst. By in-line according to the presentinvention is meant that there is a direct connection of the source ofthe exhaust gas with the zeolite and the main converter so thatundiverted passage is provided for the exhaust gas from its sourcethrough the zeolite unit and the main converter, i.e., with no by-passvalving.

In order for a zeolite to be used as an "in-line" adsorber according tothe present invention, the zeolites must have high thermal stability.For the purposes of the present invention, the degree of thermalstability is determined by measuring the surface area of the zeoliteafter exposure to a given temperature. The better the retention ofsurface area on exposure to high temperatures, the better is the thermalstablity. The specific surface area is measured by the known BETtechnique. The Cr-zeolites of the present invention, upon exposure totemperatures of up to about 1000° C. for about 6 hours, retain at leastabout 50% of the BET surface area of the zeolite at about roomtemperature, prior to exposure to the high temperature.

Zeolites with relatively low SiO₂ to Al₂ O₃ mole ratios, that is, moleratios of no greater than about 200, are very suitable for adsorbinghydrocarbons, especially low molecular weight hydrocarbons. However,generally speaking, their thermal stability is not high enough to allowthem to be used in-line with a main catalyst.

By low molecular weight hydrocarbons is meant alkanes and alkenes with 1to 6 carbon atoms. Some examples are ethylene, propylene, butadiene,pentene, and other unsaturated hydrocarbons.

The zeolites of the present invention have a SiO₂ to Al₂ O₃ mole ratioof no greater than about 200, advantageously no greater than about 100,more advantageously about 3 to about 100 and most advantageously about 3to about 20. Therefore, they are very good adsorbers for hydrocarbons.For thermal stability, the zeolite has rare earth, chromium, andcombinations of these exchanged therein to some degree. By rare earth ismeant those elements of the lanthanide series having atomic numbers 58thru 71.

Some types of zeolites that are suited to the practice of the presentinvention are Y-type, ZSM-5, beta, mordenite, and combinations of these,the combinations depending on the types of adsorption (or conversion)desired. One particularly suited zeolite is the Y-type. A Y-zeolite canbe obtained from a supplier, for example, TSZ-350 or TSZ-360 supplied byTosoh Corporation and called "Ultra-stable Y-zeolite", or CBV 712supplied by PQ Corporation.

It is critical to the practice of the present invention that the alkalicontent of the zeolite before being exchanged with chromium ions, beless than about 0.5% by weight based on the oxide, and preferably nogreater than about 0.25%. While not wishing to be bound by theory, it isbelieved that the exchange with chromium further reduces the alkali. Asa result, the purity of the product Cr-exchanged zeolite with respect toalkali is insured in order to meet the high thermal stabilityrequirements of the present invention. The low alkali zeolite can beobtained from a supplier. Or, the alkali content can be reduced bymethods known in the art, for example by subjecting a zeolite having analkali content greater than about 0.5 wt. %, to ion exchange withanother cation. It is preferred that the alkali, eg., sodium-containingzeolite be contacted with a solution having hydrogen and/or ammoniumions as exchangeable cations, ammonium ions being more easily exchanged.Thereafter, the hydrogen and/or ammonium ions are exchanged for thechromium ions. The exchanging can be done by contacting the zeolite witha solution containing the hydrogen or ammonium ions into the zeolite atan appropriate temperature, typically from about 50° C. to about 95° C.for an appropriate length of time, typically about 1 to about 24 hours.

In accordance with one embodiment, the zeolite having the low alkalicontent is then treated with Cr⁺³ and/or rare earth ions by anytechnique known in the art such as by contacting the starting zeolitewith a solution of these ions. The zeolite can be contacted more thanone time with fresh solution depending on how much Cr and/or rare earthis desired to be exchanged therein. The solution can be made bydissolving any soluble salt that yields the ions and the invention isnot limited to the nature of the solution. Examples of such saltsinclude, but are not limited to: chromium or rare earth nitrates,chlorides, sulfates, etc. The specific amounts of chromium and/or rareearths that can be exchanged into the zeolite depend on the kind andquantity of other metals that are exchanged into the zeolite and thatare desired to remain exchanged therein after the chromium and/or rareearth ions are exchanged. It also depends on the SiO₂ to Al₂ O₃ moleratio which in turn, determines the exchange capacity of the zeolite forchromium and/or rare earth ions, the capacity increasing as the SiO₂ toAl₂ O₃ mole ratio decreases.

The zeolite can be in any form depending on the application. Forexample, the zeolite can be in powder form, self-supporting geometricshapes as bead, or pellet, monoliths, eg., extruded honeycombs, etc, orbe in contact with a substrate, preferably a honeycomb substrate.

If the zeolite is in powder form, it can be slurried with the metal saltsolution. If the zeolite is in a self supporting shape, the shape can besprayed with, dipped into, or coated with the metal salt solution.

The resulting Cr and/or rare earth-exchanged zeolite is then washed ifnecessary, usually with deionized water to remove the excess chromiumsolution from the surfaces of the zeolite.

The Cr and/or rare earth-exchanged zeolite can be dried prior to theheat-treating step, at about 100 to about 110° C. in air for about 2 toabout 24 hours.

The Cr and/or rare earth-exchanged zeolite is heat treated to stabilizeit and produce the thermally stable zeolite. Heat treating temperaturesare usually about 400° to about 600° C. Heat treating times aretypically about 1 to about 24 hours and are carried out typically inair.

One technique for making Cr-zeolite, although it is to be understoodthat the invention is not limited to this technique, is as follows. Thestarting zeolite, eg., a Y-zeolite in the H⁺ form is contacted with asolution of chromium nitrate at an elevated temperature, for example atabout 50° to about 95° C. for a period of time of about 2 to 4 hours.Proportions of zeolite and chromium can vary depending on how muchchromium is desired to be exchanged. However, typically, about 100ml toabout 1000ml, of a solution of about 0.01 to about 0.5 molar chromiumsalt solution is mixed with about 10 to about 500g of the zeolite. Thezeolite is then removed from the resulting liquor by techniques such asfiltration or decantation, and is then washed a number of times,typically about 2 to 6 times, with hot deionized water to wash away theexcess chromium salt solution from the surfaces of the zeolite. In orderto increase the Cr loading in the zeolite, the zeolite after separationfrom the liquor and usually prior to washing, can be recontacted with afresh chromium salt solution. These steps of contacting (and washing, ifdesired), can be repeated until a sufficient amount of Cr is exchangedinto the zeolite. The washed zeolite is then dried typically at about100 to about 110° C. in air for about 2 to about 24 hours. The driedzeolite is then heat-treated in air at typically about 400° to about600° C. for a sufficient time to stabilize the Cr in the zeolite andproduce the thermally stable zeolite of the present invention.

The minimum amount of Cr in the zeolite that is required to impartthermal stability is a function of the zeolite SiO₂ to Al₂ O₃ moleratio, the kind and amounts of other metals present in the zeolite, etc.In general, the more Cr in the zeolite, the greater is the degree ofthermal stability imparted to the zeolite. The levels of Cr in thethermally stable zeolite are greater than about 0.5 wt. %,advantageously greater than about 1.0 wt. % and even more advantageouslygreater than about 2.0 wt. %. The maximum amount of Cr that can beexchanged into the zeolite is limited by the zeolite exchange capacity.

FIG. 1 shows that at room temperature the starting zeolite has a BETsurface area of greater than about 600 m² /g. Upon heat treatment atabout 1000° C. for about 6 hours, it suffers significant loss in surfacearea, of less than about 200 m² /g. On the other hand,chromium-exchanging this zeolite permits a significantly higher part ofthe original surface area (>50%) (and therefore catalytic activity) tobe retained.

Additionally, the zeolite can have other catalyst metals exchangedtherein, such as base metals, eg., copper, or noble metals, eg., Pt, Pd,Rh or combinations of these. The noble metals catalyze combustion of anycarbon ("coke") combustion that is deposited on the zeolite by thedecomposition of the hydrocarbons in the exhaust gas. The level of noblemetal is no greater than about 0.5% by weight of the Cr and/or rareearth-zeolite.

In accordance with one preferred embodiment, the zeolite is contactedwith a substrate. In this technique, a slurry of the zeolite iscontacted with a substrate to form a green coating thereon which is thendried and heat-treated. The slurry contains other components such asbinders, and dispersing agents, etc, as is known in the art. Somebinders are aluminum oxide, most preferred of which is the precursorboehmite, other precursors of aluminum oxide, e.g., aluminum nitrate,and silica, titania, zirconia, rare earth oxides, e.g., ceria, etc, andtheir precursors.

Some typical compositions are in percent by weight 0 to about 50methylcellulose, 0 to about 50 silica, silica gel, or silica precursors,0 to about 50 Al₂ O₃ from boehmite, aluminum nitrate, or alumina sol,and about 50 to about 95 of the zeolite.

More preferred compositions are in percent by weight 0 to about 5methylcellulose, 0 to about 30 silica, silica gel, or silica precursors,0 to about 30 alumina from aluminum nitrate, 0 to about 15 alumina fromboehmite, and about 70 to about 90 being the zeolite.

The zeolite can be blended with about 0.5 to about 2.0% methylcellulose(Dow A4M). In each case a slurry is formed in a suitable machine such asa ball mill, mix-muller, or double-arm mixer by admixing with a liquidmedium optionally containing about 0.01 to about 1.0% by weight surfaceactive agent such as Airco's Surfanol 485. The liquid medium is added tothe solids to obtain about 25 to about 60 wt. % solids content. Thepreferred liquid medium is water, however organic liquids in combinationwith water can also be used, for example, isopropyl alcohol water.Organic liquids by themselves can also be used, e.g., toluene or xylene.Optionally the slurry can have surfactants such as, Surfanol^(R).Application of the slurry to the substrate can be done by any convenienttechnique such as dipping, or spraying, depending on size and geometryof the substrate, and the invention is not limited to any technique.However, most typically it is done by dipping the substrate in theslurry followed removing the excess slurry by blowing it off. Thesubstrate is then dried to remove the water. The dipping and drying isrepeated if necessary until the desired amount of slurry components areapplied.

The green coated substrate is heat treated at sufficient temperature fora sufficient time to form the zeolite as a washcoat on the substrate,and to bond the particulates of the washcoat to the substrate and toeach other. The heat treating conditions vary with the specific slurrycomponents, size and configuration of the substrate, and otherprocessing conditions. However, in general the heat treating conditionsare about 400° C. to about 700° C., and preferably about 500° C. toabout 650° C. for about 3 to about 6 hours.

It is to be understood that the invention is not limited to the natureof substrate materials. However, the substrate is most desirably made ofany material that is suitable for high temperature applications. Somepreferred materials are those that include as a predominant phase:ceramic, glass-ceramic, glass, high surface area-high temperature stableoxides, metal, and combinations thereof. By combinations is meantphysical or chemical combinations, eg., mixtures or composites. Somesubstrate materials that are especially suited to the practice of thepresent invention, although it is to be understood that the invention isnot limited to these, are those made of cordierite, mullite, clay, talc,zircon, zirconia, spinel, alumina, silica, lithium aluminosilicates,alumina silica, feldspar, titania, fused silica, nitrides, carbides,borides, eg., silicon carbide, silicon nitride or mixtures of these.Some typical ceramic substrates are disclosed in U.S. Pat. Nos.4,127,691 and 3,885,977. Those patentss are herein incorporated byreference as filed. Some preferred metal substrates are stainless steelsand iron group metal based bodies, (Fe, Co, Ni) such as, for example, Feand Cr and/or Al bodies with optional additions of various metals and/oroxides for various properties. Some typical metal or metal alloy bodiesare disclosed in U.S. Pat. Nos. 4,758,272 and 4,992,233 and U.S.application Ser. No. 07/767,889, now U.S. Pat. No. 5,427,601 filed Sep.30, 1991 European patent application publication no. 488716A1, publishedMar. 6, 1992). Those patents and application are herein incorporated byreference as filed. Electrically heated porous or non-porous substratesare also suitable.

The substrates can be of any size and shape suitable to the application.Preferred substrates are honeycomb structures. Some examples ofhoneycombs produced by the process of the present invention, although itis to be understood that the invention is not limited to these, arethose having about 94 cells/cm² (about 600 cells/in²), about 62cells/cm² (about 400 cells/in²), or about 47cells/cm² (about 300cells/in²), those having about 31 cells/cm² (about 200 cells/in²), orthose having about 15 cells/cm² (about 100 cells/in²). These bodies aremade preferably of, but not limited, to materials which when fired formcordierite. Typical wall thicknesses in catalytic converterapplications, for example, are about 6 mils (about 0.15 mm) for 400cells/in² (62 cells/cm²) honeycombs. Wall thicknesses range typicallyfrom about 0.1 to about 0.6 mm, (about 0.004" to about 0.025"). Theexternal size and shape of the body is controlled by the application,e.g. engine size and space available for mounting in an automotiveexhaust treatment application.

The substrate can have any degree of porosity from low to high. Forexample, typically the wall porosity ranges from about 0% by volume tohigher values which are determined by practical limits depending on thecomposition of the substrate and the intended application. For example,in metal monoliths, the open porosity is typically about 1 to about 2%by volume, although it can be as high as about 40%. For ceramicmonoliths, the open porosity is typically about 25% to about 50% byvolume.

The main catalyst can be any catalyst that converts NO_(x), CO, andhydrocarbons to innocuous products and the invention is not intended tobe limited to any specific NO_(x), CO, and hydrocarbon conversioncatalyst. Some preferred main catalysts are for example, noble metal aseg, Pt, Pd, Rh, or combinations thereof on alumina, ceria, lanthana,zirconia, yttria, or combinations thereof. It is especially preferred touse a three-way catalyst. Some typical three-way catalysts which areespecially suited to the practice of the present invention for autoexhaust conversion are Pt on ceria-alumina combined with Rh on zirconia.The Pt-ceria-alumina and the Rh-zirconia can be combined and applied atonce, as in a single coating or they can be applied in separatecoatings. Another suitable catalyst is Pt/Pd/Rh on gamma alumina with arare earth oxide such as ceria.

The zeolite, in any convenient form is directly connected with theexhaust gas, and upstream of the main catalyst. Depending on otherdemands of the system, it is possible to include additional zeoliteunits connected to the original zeolite unit or the main catalyst.

In accordance with a preferred embodiment, the zeolite is positionedupstream of the main catalyst, so that the exhaust gas flows directly tothe zeolite, is contacted with the zeolite and then flows from thezeolite directly to the main catalyst. For example, the zeolite inpowder form, pellets, beads, etc. can be in a container having an inletand outlet end through which the exhaust gas passes to come in contactwith the zeolite. The zeolite can be in a self supporting shape havingits inlet and outlet ends connected by conduits to the exhaust gas andto the main catalyst respectively. One preferred self supporting form isa honeycomb which is made typically by extrusion. The exhaust gas passesthrough the cells of the honeycomb from inlet to outlet end. Or thezeolite can be in contact with a substrate as described previously whichhas inlet and outlet ends connected to the exhaust gas and main catalystrespectively. One preferred substrate is a honeycomb substrate. Exhaustgas is carried through a conduit to the substrate, passes through theopen ended cells of the honeycomb to come in contact with the zeolite,and exits the substrate and is carried by a conduit to the maincatalyst. The zeolite and main catalyst can share one substrate with thezeolite being on upstream end and the main catalyst being on thedownstream end so that the exhaust gas flows to the substrate where itis first contacted with the zeolite and then with the main catalystafter which it exits the substrate.

Optionally, a means for injecting air into the zeolite adsorber such asan air injection pump may be present. The function of such an airinjection system is to regenerate the zeolite adsorber after the maincatalyst has reached its operating temperature. Alternately, the zeolitecan be regenerated by the exhaust gas itself, by operating the engineunder fuel lean or oxidizing conditions, for a short period of timeafter the main catalyst has reached light-off temperature, so that thereis a sufficient excess of oxygen in the exhaust. The objective of suchregeneration is to oxidize any carbon or "coke" deposited on the zeolitesurfaces, so that the capacity for adsorption is recovered (for the nextstart-up) cycle.

FIG. 2 is a schematic diagram showing one type of arrangement of theimproved exhaust gas conversion system of the present invention. Thesystem (10) is made of a first conduit (12) which is connected to theinlet end (14) of the zeolite conversion unit (16) which is preferably azeolite in contact with a honeycomb substrate, such as a ceramic,preferably cordierite substrate, or a zeolite in the form of ahoneycomb. An exhaust gas mixture such as auto exhaust, is conveyed inits entirety through the first conduit to the inlet end of the zeoliteunit, into the zeolite unit to undergo hydrocarbon adsorption. A secondconduit (18) is connected at one end to the outlet end (20) of thezeolite unit. The other end of the second conduit is connected to theinlet end (22) of main converter (24). The exhaust gas in modified formis conveyed through the second conduit to the second unit to undergoconversion of NO_(x), Co, and hydrocarbons. The second conduit isconnected only to the outlet end of the zeolite unit and the inlet endof the main converter unit. An exit conduit (26) is shown connected tothe outlet end (28) of the main catalyst unit. The main catalyst unit asshown is typically a honeycomb substrate having the catalyst in contacttherewith, such as for example, by washcoating. The converted gasmixture passes through the exit conduit to ambient atmosphere. Thearrows indicate the direction of flow through the apparatus.

It is to be understood that the respective conduits can be connected tothe respective units by any means known in the art. It is to beunderstood also that respective sizes and lengths of the conduits andunits can vary depending on the application and that the invention isnot limited to any sizes or size relationships of conduits and catalyzedunits.

FIG. 2 shows the units in a conventional canister or can shown as (30)with the inner surface shown as 30a, as is used in automotiveapplications. The units are held fixed in the canister by conventionalmeans such as, for example, by metal mesh, or ceramic mats, etc.Refractory fibrous material is often used to prevent passage of gasesbetween the units and the canister. The various sections of the canserve as the respective conduits and for the purposes of the presentinvention are considered to be connected to the units.

The units can be in separate cannisters. FIG. 3 is a schematic diagramshowing an exhaust gas conversion system (40) in which the zeolite unit(42) and the main catalyst unit (46) are in separate cannisters (44) and(48) respectively, and connected to one another and to an exhaust gassource in essentially the same manner as shown in the system in FIG. 2.

To more fully illustrate the invention, the following non-limitingexamples are presented. All parts, portions, and percentages are on aweight basis unless otherwise stated.

EXAMPLE

An (NH₄)⁺ form Y-zeolite such as CBV-712 zeolite supplied by PQcorporation having a SiO₂ to Al₂ O₃ mole ratio of about 12 is exchangedwith Cr⁺³ by contacting it with a Cr(NO₃)₃ solution, and refluxing themixture at about 80°-95° C. for at least about 2 hours. The Cr-exchangedzeolite is thoroughly washed and separated from the liquids, dried atabout 100° C. for at least about 8 hours in air, and heat-treated atabout 400°-550° C. for about 6-12 hours. The Cr-exchanged zeolite andboehmite alumina binder such as Dispersal^(R) from Condea Chemie aremixed in a ratio of about 85 to 15. The mixture is slurried withsufficient deionized water such that about 50 wt. % solids is obtained.This slurry is mixed with Al₂ O₃ media of about 3 times the weight ofthe solids in the slurry and rolled in a suitable Nalgene container on aset of rolls for about 2 hours. The pH of the slurry is then adjusted toabout 3.5 with dilute nitric acid. To the slurry is added 0.5 wt. % (ona solids basis) of a surfactant such as Surfanol^(R) supplied by AirProducts and Chemicals Co. The viscosity of the slurry is measured usinga Brookfield viscometer. A viscosity of about 5-15 centipoise isgenerally desirable. If the viscosity is too high, water is slowly addedto dilute the slurry to the desirable range. A cordierite honeycombsubstrate having about 400 cells/in² (about 62 cells/cm²) with about,0.006" (about 0.015 mm) thick walls is used to support the zeolite. Thezeolite slurry is washcoated by dipping the honeycomb substratecompletely into the slurry, removing it, blowing the excess slurry offwith air and drying it at about 100° C. for about 30-60 minutes. Theweight of the washcoat on the substrate is monitored, and the dippingand drying procedure is repeated until about 35-45% washcoat (i.e.,about 35-45 g of washcoat per about 100 g of substrate) is deposited onthe substrate. Following this, the washcoated substrate is calcined inair at about 500-600° C. for about 2-6 hours. The size of the substratefor the zeolite adsorber unit is a function of the volume of exhaustgases to be treated, and therefore, a function of the engine size.Substrate volumes can range from about 0.5 to about 4.0 liters. TheCr-zeolite washcoated substrate is packaged in a steel can usingrefractory fiber or mat insulation according to techniques known in theart. A three-way catalyst (main catalyst) is prepared according totechniques known in the art. Typically, a Pt/Pd/Rh catalyst on about 70%gamma Al₂ O₃ and about 30% CeO₂ washcoated on a cordierite honeycomb isused. The (Pt+Pd) : Rh ratio is typically about 5:1 to 10:1, with aPt:Pd ratio of about 1:1 to about 3:2. The total precious metal loadingis about 30-60 g/ft³ of substrate, (about 10-21 g/l). This catalystcoated substrate is also packaged in a steel can with refractory fiberor mat insulation using techniques known in the art. The Cr-zeoliteadsorber and main catalyst units are placed in the exhaust stream of aninternal combustion engine such that the adsorber is upstream and themain catalyst is downstream. The engine exhaust port, the adsorber, andthe main catalyst are all connected directly, i.e., without any valvingor by-pass lines. When the engine is turned on from a cold condition,the exhaust initially contains an excess of hydrocarbons fromuncombusted fuel. The Cr-zeolite adsorbs a significant proportion of thecold-start hydrocarbon emissions ("cold-start" emissions are defined asthe emissions during engine start-up from ambient temperature, that areemitted prior to the main catalyst reaching its light-off temperature,about 300°-400° C.). This time period is typically about 60-180 secondsafter the engine has been turned on. In particular, the light alkenes,such as ethene, propylene, etc., are very effectively adsorbed,especially after the adsorber reaches a temperature of about 70-90° C.Once the main catalyst reaches its light-off temperature, the maincatalyst starts to very effectively convert CO and NO_(x) emissions.Furthermore, by this time, the zeolite adsorber has reached temperaturesof about 300°-400° C. and begins to desorb some of the adsorbedhydrocarbons. The desorbed hydrocarbons can include cracked hydrocarbonsand/or oxides of carbon such as CO, and CO₂. At this time, the zeoliteadsorber is effectively purged of adsorbed hydrocarbons as well as anycarbonaceous deposits from the hydrocarbon adsorption (generallyreferred to as "coke") by one of two methods: (1) an air injection pumpconnected to the inlet of the adsorber is turned on for about 30-120seconds to provide the excess oxygen required for regeneration, or (2)the engine stoichiometry is switched to a "lean" operation for about30-120 seconds so that there is sufficient excess oxygen in the exhaustto regenerate the zeolite. In either case, Cr in the zeolite is a veryeffective catalyst for decoking the zeolite (oxidation of carbon) andvirtually the entire capacity of the zeolite adsorber is recovered bythis process. The exhaust gases from the regeneration as well as theengine exhaust now contain CO, NO_(x), and hydrocarbons (among othernon-regulated, non-toxic gases) which are effectively converted toinnocuous products by the main catalyst since it is at its operatingtemperature.

It should be understood that while the present invention has beendescribed in detail with respect to certain illustrative embodimentsthereof, it should not be considered limited to such but may be used inother ways withoud departing from the spirit of the invention and thescope of the appended claims.

What is claimed is:
 1. A method for removing at least one of thepollutants NO_(x), CO, and hydrocarbons from an exhaust gas, said methodcomprising contacting said exhaust gas in undiverted flow first with azeolite suitable for adsorption of hydrocarbons, and thereafter with amain catalyst suitable for conversion of NO_(x), CO, and hydrocarbons toinnocuous products, at a temperature ranging from cold-starttemperatures to light-off temperatures, wherein when the temperature isin the cold-start range and the exhaust gas contains hydrocarbons, thezeolite adsorbs the hydrocarbons, and when the temperature is in thelight-off range, and the exhaust gas contains NO_(x), CO, andhydrocarbons, the main catalyst converts the NO_(x), CO, and thehydrocarbons to innocuous products, said zeolite being selected from thegroup consisting of Y zeolite, Beta, ZSM-5, mordenite, and combinationsthereof, and having a SiO₂ Al₂ O₃ mole ratio of no greater than about200, said zeolite being ion exchanged with chromium.
 2. A method ofclaim 1 wherein said zeolite is Y-zeolite.
 3. A method of claim 1wherein said ratio is no greater than about
 100. 4. A method of claim 3wherein said ratio is about 3 to about
 100. 5. A method of claim 4wherein said ratio is about 3 to about
 20. 6. A method of claim 1wherein the alkali content of said zeolite is less than about 0.5% byweight based on the alkali oxide.
 7. A method of claim 6 wherein saidalkali content is no greater than about 0.25% by weight.
 8. A method ofclaim 1 wherein said zeolite is in contact with a substrate.
 9. A methodof claim 8 wherein said substrate is a honeycomb structure.